Chapter 6 - Visual System and How We See

Light and Properties of Light

Light can be thought of as particles (photons) or as waves of energy

-Found on the electromagnetic spectrum, only part that we can view (visible light)

-Wavelength decreases and frequency increases as you move to the left, as you move to the right wavelength increases and frequency decreases

-Higher frequency = more energy

Light spans wavelength between 380nm (low wave, higher freq) to 780nm (high wave, low freq)

-Infrared waves are too long for humans whilst ultra violet wave is too short, so we cannot see them

Two key properties of light are Wavelength (important in perception of color) and intensity (important in perception of brightness, amplitude of the wave)

Light behaves as a wave as it moves through the eye, then behaves as particle when it reaches the retina

Properties of Light: Interactions

Light can be:

1. Transmitted (from one medium to another)

2. Reflects

3. Refracted (if light is shone on a non-90 degree angle)

4. Diffraction (equally spreading into all directions)

5. Absorption

6. Scattering

Light Entering the Pupil

Amount of light reaching the Retina is regulated by contractile tissue (Irises) which also give our characteristic colour

-As light enters, it is refracted by the internal components (behaving like a wave)

-Light passes through the cornea, then enters through the pupil (hole in the iris), adjustment of pupil size in response to change in illumination is a compromise between:

1. Sensitivity (ability to detect presence of dimly lit objects), and

2. Acuity (ability to see details of objects)

-When illumination is high, sensitivity is less important so our pupils constrict

-Constriction of the pupil results on image falling on retina being sharper, creating a greater depth of focus (2-3mm in diameter)

-When illumination is too low to activate receptors, pupils dilate to let more light (sacrifices acuity and depth of focus), 7mm in diameter

Lens and the Ciliary Muscles

Lens is located behind the pupil, focuses incoming light onto the fovea

-Ciliary muscles and ligaments that hold each lens in place; when we focus on a distant object the lens is flattened (relaxation of ciliary muscles, tensing of ligaments);

-when we focus on a close object the lens assumes its natural cylindrical shape (contraction of ciliary muscles, relaxation of ligaments)

Accommodation - Process of adjusting configuration of lenses to bring images into focus onto the retina

-Another mechanism for depth perception

Eye Position Advantage

Having one eye on each side allows us to see in any direction without moving heads

-Eyes in front of the head provide depth perception, allowing predators to determine distance of prey (humans, owls, etc)

-Animals with eyes on the side of the head provide greater field of vision, allowing them to see predators from most directions

-Having eyes mounted side-by-side sacrifices ability to see behind with the ability to see what is Infront through both eyes simultaneously

Binocular Disparity

Difference in position of the same image on the two retinas; greater for close objects than for distance objects

-Low binocular disparity for further objects, high binocular disparity for close objects

-Binocular disparity provides depth perception (higher disparity = lower distance, low disparity = greater distance)

Convergence

When an object is located further away, convergence is low

-Closer objects as higher convergence (inward rotation of the eye)

Another mechanism in perceiving depth

Five Types of Neurons in the Retina

Photoreceptors - Receive light input, divided into cones and rods

Horizontal Cells - Retinal neurons who specialized function is lateral communication

Bipolar Cells - Neurons that form the middle layer of the retina

Amacrine Cells - Retinal neurons specialized for lateral communication
Retinal Ganglion Cells - Retinal neurons whose axons leave the eyeball and form the optic nerve, communicate via synapses and electrically via gap junctions

Photoreceptor → Bipolar Cell → Retinal Ganglion Cell form the serial connection in the Retina

Amacrine and Horizontal cells form intermediatiary connections in the Retina (lateral connections) in the serial connection

Lateral Communication

Communication across major channels of sensory input

-Performed by Amacrine and Horizontal cells

Inside-out Arrangement of Retina

Light reaches the receptors only after passing through other layers

-The neural message is then transmitted back out through those layers to the retinal ganglion cells

Creates two visual problems

1. Incoming light is distorted by retinal tissue that it passes through

2. Creates a Blind Spot - Area on the retina where bundle of axons from retinal ganglion cells leave the eye as the optic nerve, no photoreceptors are present

Fovea

0.33 cm in diameter identation at the centre of the retina at the area specialized for high-acquity vision (fine details)

-Thinning of the retinal ganglion cell layer at the fovea reduces distortion of incoming light

Completion

Visual system’s automatic use of information obtained from receptors around the blind spot to create perception of the missing portion of retinal image

Receptors of the Retina

Cones - Cone-shaped visual receptors, mediate high acuity (fine-detailed) color vision in good lighting

-Known as Photopic Vision (cone-mediated vision). high acuity and color

Rods - Rod-shaped visual receptors, mediates achromatic, low-acuity vision in dim lighting

-Known as Scotopic Vision (rod-mediated vision), lacks both detail and color

Duplexity Theory

Theory that cones and rods mediate different kinds of visions

Scotopic vs Photopic Vision Explanation

Scotopic system, hundreds of rods converge on a single retinal ganglion cells whereas the photopic system only a few cones converge on each retinal ganglion cells

-Therefore, dim light stimulates many rods simultaneously, output of this stimulation converge and summate onto the retinal ganglion cell

-Effects of the same dim light applied to cones cannot summate to the same degree, retinal ganglion cells may not respond to all the light

Distribution of Rods and Cones

No rods in the fovea, only cones

-Proportion of cones drops markedly at the boundaries of fovea, while there is an increase in number of rods

-More rods in the Nasal Hemiretina (half of retina next to the nose) than in the temporal Hemiretina (half of retina next to the temples)

Visual Transduction

Conversion of light to neural signals by visual receptors

-Rhodopsin - Pigment that, when exposed to continuous intense light, was bleached and lost its ability to absorb light, but when returned to the dark it regained its redness and light-absorbing capacity

Spectral Sensitivity Curve

Graph of relative brightness of lights of the same intensity presented at different wavelengths

Photopic Spectral Sensitivity Curve

Graph of the sensitivity of cone-mediated vision to different wavelengths of light

-Under photopic conditions, visual system is sensitive to wavelengths up to 560nm (so light at 500nm would have to be more intense than one at 560nm to be equally as bright)

Scotopic Spectral Sensitivity Curve

Graph of the sensitivity of rod-mediated vision to different wavelengths of light

-Visual system is sensitive to wavelengths up to 500nm, thus under scotopic conditions light of 560nm would have to be more intense than one at 500nm to be equally as bright

Purkinje Effect

Intense light red and yellow wavelengths look brighter than blue or green wavelengths of equal intensity

-in dim light, blue and green wavelengths look brighter than red and yellow wavelengths of equal intensity

-Result of the difference in photopic and scotopic spectral sensitivity

Fixational Eye Movements

Involuntary movements of the eyes (through either tremors, drifts or saccades) that occur when a person tries to fix their gaze on a single point (staring)

-Serve a critical function, enable us to see during fixation by keeping the images moving on the retina

Rhodopsin Response to Light

Photoreceptors contain photopigments that absorb energies of light

-Rods contain Rhodopsin, whereas cones have red, blue or green photopigments (one type of pigment per cone)

G-Protein coupled receptor, responds to light rather than neurotransmitter molecules

-When rods are in darkness, sodium channels are partially open, keeping rods slightly depolarized allowing a steady flow of excitatory glutamate to emanate from them (inactive)

-But when bleached by light, the resulting cascade of intracellular chemical events closes the sodium channels and hyperpolarizes the rods, reducing the release of glutamate

-Demonstrates that signals can be transmitted through inhibition (ie, reducing release of glutamate)

Retinal Geniculate Striate System

-Conducts signals from each retina to the primary visual cortex (striate cortex/V1) via the lateral geniculate nuclei of the thalamus

-90% of axons of retinal ganglion cells become part of the Retinal Geniculate Striate pathways

-All signs from the left visual field reach the right primary visual cortex, either ipsilaterally (from the temporal hemiretina) of the right eye or contralaterally (via the optic chiasm) from the nasal hemiretina of the left eye

-Opposite is true for the right visual field

Retinotopic - each level of the system is organized like a map of the retina (two stimuli presented to adjacent areas of the retina excite adjacent neurons at all levels of the system)

-Disproportionately large representation of the fovea (25% of the primary visual cortex is dedicated to the analysis of the fovea)

Lateral Geniculate Nucleus

Six-layered thalamic structure that receives input from the retinas and transmit their output to the primary visual cortex

Each lateral geniculate nucleus has six layers, three layers receive input from one eye and the other three receive input from the other

M and P Channels

Parallel channels of communication that flow through each lateral geniculate nucleus

1. Parvocellular layers - runs through the top four layers, composed of neurons with small cell bodies

• Responsive to color, fine pattern details, slow/stationary objects, input primarily from cones

2. Magnocellular Layers - Runs through bottom two layers, compoed of large cell bodies

• Respond to movement, input primarily from Rods

Contrast Enhancement

Intensification of the perception of edges

Receptive Field

Area of the visual field within which it is possible for the appropriate stimulus to influence the firing of a visual neuron

Hubel and Wiesel Experiment

Recorded the three levels of the retina-geniculate-striate system, first from retinal ganglion cells, then lateral geniculate neurons, then striate neurons of lower layer 4

-Tested the neurons with stationary spots of uncolored light, found little change in receptive fields

-When they shone the achromatic light onto parts of the receptive field of neurons in the retina-geniculate-striate pathway, they discovered two responses; either “on” firing or “off” firing" depending on location of the spot of light in the receptive field

On-Center and Off-Center Cells

On-Center Cells - respond to lights shone in the central region of their receptive fields with “on” firing, and to lights shone in the periphery of their receptive fields with inhibition, followed by “off” firing when light is turned off

Off-Center Cells - respond with inhibition and “off” firing in response to lights in the center of their receptive fields, and with “on” firing to lights in the periphery of their receptive fields

Simple Striate Cells

Neurons in the visual cortex that respond maximally to straight-edge stimuli of a particular width and orientation

Have receptive fields that can be divided into antagonistic “on” and “off” regions

-Unresponsive to diffuse light

-Similar to lower layer 4 neurons, all are monocular (receptive field in one eye, but not the other)

-Borders between “on” and “off” regions of the cortical receptive fields of simple cells are straight lines rather than circles

-Respond best to bars of light in a dark field, or dark bars in a light field, responds maximally to straight-edge stimuli

Complex Striate Cells

Neurons in the visual cortex that respond optimally to straight-edge stimuli in a certain orientation in any part of their receptive field

-More numerous than simple cells, also have rectangular receptive fields and respond best to straight-line stimuli, unresponsive to diffuse light

-Differ from simple cells:

1. Larger receptve field

2. Not possible to divide receptive field into “on:” or “off” regions

3. Many complex cells are binocular (respond to stimulation of either eye)

Binocular Complex Striate Cells

Most of the binocular cells display Ocular Dominance - respond more robustly to stimulation of one eye, than they do to the same stimulation of the other

-Some fire best when the preferred stimulus is presented to both eyes at the same time, but in slightly different positions on the two retinas (respond best to “retinal disparity”, playing a role in depth perception)

Organization of Primary Visual Cortex

Hubel and Wiesel concluded that;

1. Primary visual cortex was organized into functional vertical columns, neurons in the same column respond to stimuli applied to same are of the retina, are dominated by the same eye, and prefer the same “straight-line” angles

2. All functional columns analyze input from one area of the retina are clustered together, half receives input from left eye the other from the right eye

• Each provides neurons with preferences for straight-line stimuli of various orientations

3. Preferences of neurons became more complex as they from retina to thalamus to lower layer 4 of visual cortex, simple then complex cortical cells

• likely because neurons with simpler preferences converge on neurons with complex preferences

Retinal Ganglion Cells

20-40 distinct types of cells, each with its own receptive field

-In addition to on and off-center receptive fields, they also have receptive fields selective of uniform illumination, orientation, motion, and direction of motion

Lateral Geniculate Cells

Have receptive fields sensitive to more than just contrast (on-center and off-center receptive fields)

-Sensitive to orientation, motion, and direction of motion (similar to retinal ganglion cells)